Thin-layer porous optical sensors for gases and other fluids

ABSTRACT

A gas sensor uses optical interferents in a porous thin film cell to measure the refractive index of the pore medium. As the medium within the pores changes, spectral variations can be detected. For example, as the pores are filled with a solution, the characteristic peaks exhibit a spectral shift in one direction. Conversely, when tiny amounts of gas are produced, the peaks shift in the opposite direction. This can be used to measure gas evolution, humidity and for applications for other interferometric-based sensing devices.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims benefit of U.S. Provisional Patent Application,Ser. No. 60/533,570, entitled “Thin-Layer Porous Optical InterferometricSensors for Gases and Other Fluids,” filed Dec. 31, 2003.

STATEMENT OF FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with Government support under grant numberF33615-00-2-6059 awarded by the Human Effectiveness Directorate, AirForce Research Laboratory, Air Force Material Command, United States AirForce. The Government has certain rights in this invention.

FIELD OF THE INVENTION

The present invention relates to a thin layer electrode andmethodologies for taking spectroscopic or interferometric measurementsof a fluid. In particular, the presently disclosed technology relates toan optically reflective thin layer electrode (ORTLE) for, e.g.,spectroscopic interrogation of a solution phase within channels or poresof a film in the ORTLE.

BACKGROUND OF THE INVENTION

Since first reported in 1967, optically transparent thin layerelectrodes (OTTLEs) have been used for thin layer studies.Spectroelectrochemistry, for example, is a combination ofelectrochemical and spectroscopic techniques in which opticalmeasurements are referred to the potential of a working electrode.Thin-layer spectroelectrochemistry is possibly the simplest type ofspectroelectrochemistry and has advantages such as rapid and exhaustiveelectrolysis and small volume features. A typical application of anOTTLE is the spectroscopic study of “redox” processes; i.e., reactionsin which the acceptance of an electron (reduction) by a material ismatched with the donation of an electron (oxidation). Variousspectroscopic techniques such as luminescence spectroscopy, Fouriertransform infrared (FTIR) difference spectroscopy andultraviolet/visible/near infrared (UV/vis/NIR) have been coupled withelectrochemical techniques via OTTLEs. A variety of OTTLE designs formany purposes have been developed, and all generally operate ontransmittance principles.

BRIEF SUMMARY OF THE INVENTION

The present invention is directed to spectroelectrochemical,spectroscopic and/or interferometric analyses of a material trapped inpores of a thin layer film. More specifically, the invention is directedto an optically reflective thin layer electrode (ORTLE) that operates onreflectance principles; i.e., the ORTLE collects reflectance rather thanfacilitating transmittance through a fluid. As used herein, the termfluid is used to mean a continuous amorphous medium, matter orsubstance—e.g., a liquid, including a solution, or a gas—that tends toflow and to conform to an outline of its container and will not destroythe thin layer film.

The ORTLE is fabricated, for instance, by anodizing a thin layer ofaluminum sputtered onto a glass substrate, such as a float glassmicroscope slide, to create a 250 nm to 1000 nm thick, porous, aluminumoxide (alumina) film. The resulting alumina film is transparent andcontains channels or pores that vary from approximately 80 to 100 nm indiameter with depths of approximately 250 nm to 1000 nm. The thicknessof the alumina film—and thus the depth of the pores—can be altered bycontrolling the thickness of the original aluminum film. Althoughalumina is provided by way of an enabling example, any porous membranewith channels or pores to trap the fluid for spectroscopic,interferometric, or spectroelectrochemical measurements can be used topractice the invention.

In one aspect of the invention, a thin film of gold is sputtered atopthe alumina film. The gold layer adheres to surface points of thealumina film such that the gold layer remains porous to allow a solutioninto the pores of the alumina film while remaining optically thick andreflective. The gold layer is filled with holes each having a diametersubstantially less than the wavelength of visible light. As discussedbelow, the gold layer can serve as an electrode but is not limited tosuch use. Furthermore, other metals and materials with reflectiveproperties can be substituted for gold and thus used to practice theinvention.

As described in detail herein, the ORTLE interrogates a very thin filmsample based on porous alumina. Through the use of a combination ofspecular reflectance spectroscopy and chronoamperometry, thespectroelectrochemical study is confined to that solution containedwithin the pores of the ORTLE. Specifically, spectroscopy interrogates asolution phase within the pores of the alumina film between theelectrode face and a window behind it. Reflectance measurements are thusmade through the glass slide but do not interrogate a surrounding bulksolution.

According to exemplary experiments described herein, the reflectancemeasurements show spectral features that shift with the opticalproperties of the material filling the pores of the alumina film. Thestability of the ORTLE spectrum and its origin are described by examplesbelow to show how an applied potential affects the observed spectrum ina simple solution. For instance, a series of experiments in which thepotential of the ORTLE is stepped negatively to various values in anaqueous sodium sulfate solution shows that interference fringes shiftmeasurably in the ORTLE spectrum at potentials several hundredmillivolts positive of the potential at which gas evolution was visibleto the naked eye.

According to a particular aspect of the invention, a method is providedfor analyzing matter such as by interferometric, spectroscopic, and/orelectrochemical and optical spectroscopic (spectroelectrochemisty)techniques. The method includes the steps of (a) introducing a liquid orgaseous matter into an optically reflective thin layer electrode, theelectrode including a transparent base substrate with alumina filmdisposed thereon, the alumina film defining a plurality of porestherein, and a gold film disposed on the alumina film such that aquantity of the liquid or gaseous matter can enter at least one of thepores; (b) applying a potential to the gold film such that the quantityof the liquid or gaseous matter in the pores is isolated from aremaining bulk of the liquid or gaseous matter disposed about theelectrode; and (c) directing light from a source into the electrode fromproximate the base substrate in a direction of the gold film, the foldfilm under the potential configured to reflect the light into thequantity of the liquid or gaseous matter in the pores for analysis ofthe reflected light.

According to the method, the liquid or gaseous matter is selected fromthe group consisting of potassium ferricyanide, sodium sulfate, waterand solutions thereof. The liquid or gaseous matter can be a solution of0.01M ferricyanide, 0.05M sodium sulfate, and deionized water. Thetransparent base substrate is made of glass. The applied potential isbetween +0.4V to −1.5V and can be held for between about 200 seconds toabout 400 seconds.

According to the method, the light can be directed at the base substrateat about a 45° angle, and may further include the step of monitoring thereflected light in the quantity of the liquid or gaseous matter isolatedin the pores by reflectance spectroscopy.

According to another aspect of the invention, an optically reflectivethin layer includes a transparent base substrate; a film disposed on thebase substrate, the film defining a plurality of pores therein; and areflective material disposed on the film such that the pores are exposedto atmosphere, the reflective material having a specular surface forreflection of light into the pores for taking measurements of a fluidisolated therein from the atmosphere.

In this example, the pores are from 80 to about 100 nm in diameter andfrom 250 nm to about 1000 nm deep, more particularly about 750 nm, andhold a quantity of fluid when the optically reflective thin layer isimmersed in the fluid. The fluid can be selected from ferricyanide,sodium sulfate, water, a gas, or combinations of these. Moreparticularly, the fluid can be a solution of 0.01M ferricyanide, 0.05Msodium sulfate, and deionized water.

Also in this aspect, the gold film will reflect light transmitted from adirection of the base substrate into the quantity of fluid in the poreswhen a potential is applied. The quantity of fluid is monitorable byspecular reflectance spectroscopy using the reflected light, or byspectroelectrochemical analysis or interferometric analysis orcombinations of these methodologies. A detector arranged at about 90° tothe reflected light is provided for monitoring and taking measurements.

In yet another embodiment of the present invention, a thin layerelectrode includes a transparent base substrate with a porous thin filmdisposed on the base substrate, and a material disposed on the thin filmsuch that the pores are exposed to atmosphere containing a fluid.

In an aspect of this embodiment, the material is a gold film, which willisolate a portion of the fluid in the pores away from a remainder of thefluid in the atmosphere when a potential is applied. The gold film isalso optically reflective when the potential is applied to redirect anincident light beam into the pores as the light beam passes through thetransparent base substrate into the pores in a direction of the goldfilm. Additionally, the gold film has a nanostructured face disposedaway from the thin film, which filters scatter-causing particlessuspended in the fluid.

Similar to the foregoing aspects of the invention, the transparent basesubstrate can be glass and the thin film can be alumina. By way ofexample, the thin film alumina has a depth of 250 nm to about 1000 nmand exhibits pores of about 80 nm to about 100 nm in diameter and from250 nm to about 1000 nm deep. Also, the fluid in this aspect is asolution of ferricyanide, sodium sulfate, and water, more particularlydeionized water, or a gas, or combinations of these components.

In another aspect of the invention, a thin layer apparatus for fluidanalysis is provided with a transparent base substrate, and a thin filmsputtered on the base substrate. The thin film has a plurality of pores,each of which has a diameter of 80 nm to about 100 nm and depths of 250nm to about 1000 nm. Similar to the foregoing embodiments, the poresisolate a portion of a fluid from a remainder of a bulk fluid in whichthe thin layer apparatus is immersed.

A material can be layered on the thin film in this aspect such that thepores remain open in communication with the fluid. The material has aspecular surface to reflect into the pores an incident beam entering thepores through the transparent base substrate in a direction of thematerial such that the isolated fluid in the pores can be analyzed byspecular reflectance spectroscopy using the reflected light, or byspectroelectrochemical analysis or interferometric analysis orcombinations of these methodologies.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other aspects and advantages of the present invention areapparent from the detailed description below in combination with thedrawings, in which:

FIG. 1A is a schematic elevational view of an ORTLE according to anaspect of the present invention;

FIG. 1B is a SEM image of porous alumina covered with a layer of gold asin FIG. 1A, particularly showing unsealed pores in the porous alumina;

FIG. 2 is a schematic of a cell in which the ORTLE as in FIG. 1 ismounted;

FIG. 3 is a schematic plan view of the cell as in FIG. 2;

FIG. 4 is a graph showing cyclic voltammograms (CV) obtained for astandard gold electrode and the ORTLE as in FIG. 2 immersed in anelectrolyte solution with a potential applied;

FIG. 5 is a graph showing transmission spectra for a glass slide, aglass slide coated with an alumina layer, and the ORTLE as in FIG. 1;

FIG. 6 is a graph showing specular reflectance spectra including a redshift observed after introduction of a 0.05M sodium sulfate solution andchanges in the spectra over time;

FIG. 7 is a schematic plan view of optical path-lengths of incidentlight from air through a glass substrate and alumina layer with tworeflective interfaces resulting from unanodized aluminum and goldcoating;

FIG. 8 is a graph showing a CV obtained for an ORTLE according toanother aspect of the invention;

FIG. 9 a is a graph showing an effect of potential on a specularreflectance spectrum concentrating on a peak at approximately 755 nm;and

FIG. 9B shows a decrease in magnitude of the reflectance at 780 nm withincreasing negative potential, particularly showing by dashed line a drystate of the ORTLE at 780 nm according to another aspect of theinvention.

DETAILED DESCRIPTION OF THEE INVENTION

Detailed reference will now be made to the drawings in which examplesembodying the present invention are shown. The detailed description usesnumerical and letter designations to refer to features of the drawings.Like or similar designations of the drawings and description have beenused to refer to like or similar parts of the invention.

The drawings and detailed description provide a full and detailedwritten description of the invention and of the manner and process ofmaking and using it, so as to enable one skilled in the pertinent art tomake and use it, as well as the best mode of carrying out the invention.However, the examples set forth in the drawings and detailed descriptionare provided by way of explanation of the invention and are not meant aslimitations of the invention. The present invention thus includes anymodifications and variations of the following examples as come withinthe scope of the appended claims and their equivalents.

ORTLE Design and Concept

An optically reflective thin layer electrode (ORTLE) generallydesignated by the reference numeral 10 is broadly depicted in FIG. 1A.In this example, a 500 nm aluminum film was sputtered substantially ontoa base substrate such as a transparent float glass slide 12, anodizedand subsequently converted to porous alumina 14. Specifically, a 750nm-thick, transparent, alumina film 14 results due to conversion fromaluminum metal to alumina (aluminum oxide). The alumina film 14 wassubsequently coated for 210 seconds with gold using a sputtering system(not shown) such as a CRC-100 sputtering system available from PlasmaSciences Inc. of Lorton, Va. The sputtering system produced a gold film16 approximately 100 nm thick.

Although the film 16 appears in FIG. 1A to be an optically thick goldfilm, scanning electron microscopy (SEM) laboratory studies have shownit insufficient to seal the relatively larger pores of a porous aluminamembrane. Specifically, a SEM image in FIG. 1B shows that channels orpores 18 of the alumina film 14 created by the above conversion processalso remain open when coated with gold in the foregoing manner.

The gold film 16 possesses a mirror finish on its face 16 a—the exposedside opposite the porous alumina 14. Despite this and the apparentcontinuity of the film 16, it is highly porous thus exposing thechannels 18 in the underlying alumina 14 to atmosphere. A reverse side16 b of the gold film 16, however, does not show a highly reflectivefinish, although it presents a mostly specular surface. As brieflyintroduced, other metals or materials exhibiting reflective qualitiescan be substituted for the exemplary gold film 16.

When exposed to a bulk solution S (see FIG. 2), the channels 18 arefilled with a quantity or portion of the solution S. Light can passthrough the optically transparent glass slide 12 used as a support andthrough the fluid-filled alumina 14, but is reflected by the porousmetal overlayer 16. If a potential is applied to the gold film 16, anysolution S changes that occur within the pores 18 can be monitored byspecular reflectance spectroscopy. The ORTLE design advantageouslyrequires no special auxiliary or reference electrodes and no specialelectrode configurations.

Those skilled in the art will immediately recognize and appreciate thatdeeper pores, e.g., greater than about 750 nm, will provide a strongerspectrum of the material in the pores 18, but will take longer toequilibrate with material outside the pores 18 since spectrum strengthincreases linearly with length. Conversely, shallower pores, e.g., lessthan about 750 nm, will equilibrate faster for faster measurements butwill provide a weaker spectrum since equilibration time increases as thesquare of length.

Cell Design and Set-Up

The present cell design is used as a window into the bulk solution S,since only the solution S in the alumina pores 18 is interrogated. Asshown in FIG. 2, the ORTLE 10 is mounted in an exemplary cell 20. Thecell 20 is constructed from a Teflon®-brand material and contained in arectangular window 22 (e.g., 75×75×0.5 mm) to which the thin layerelectrode 10 is attached. The ORTLE 10 is positioned with the goldsputtered surface or side 16 facing inwards as shown and held in place,for instance, by eight screws 24 a-h, which can be tightened to avoidleaking of the solution S.

As described by example operation below, the ORTLE 10 can be used in theforegoing arrangement as a working electrode in the cell 20 forspectroelectrochemical analysis. However, the skilled artisan willrecognize that the ORTLE 10 need not be used an electrode nor is thearrangement limited to spectroelectrochemical analysis. For instance,the ORTLE 10 can be used for spectroscopic and interferometric analysesof material trapped in the pores 18 of the alumina 14. As presentedbelow, while an interference pattern can shift to indicate a change in arefractive index of the material filling the pores 18, one skilled inthe art can also perform infrared or UV-visible spectroscopymeasurements for identification of the pore fluids. Thus, themeasurements may not be purely interferometric in nature and may includeabsorption properties of the fluid.

With reference to FIGS. 2 and 3, the cell 20 is mounted on an aluminumbase 27, which is placed inside a spectrometer 26. The height of thisaluminum base 27 is selected to place the ORTLE 10 in a path of anincident light beam L_(I) emanating from a light source L_(S). With theORTLE 10 so attached, the cell 20 is filled with the desired solution S,into which an auxiliary electrode 28 and a reference electrode 30 areinserted. In this example, the auxiliary electrode 28 is platinum (Pt)gauze and the reference electrode 30 is Ag/AgCl/sat: NaCl, availablefrom BAS of West LaFayette, Ind. Also by way of example, an OL 750-75MAAutomated Goniospectroreflectance Attachment, available from OptronicLaboratories, Inc. in Orlando, Fla., is suitable for use as thespectrometer 26.

As shown in FIG. 3, the cell 20 is illuminated by the light sourceL_(S), such as a tungsten quartz-halogen lamp (150 W) and a 750 M-SMonochromator combination. This light source combination L_(S) hasselectable wavelengths in the range of 280-1100 nm. The Monochromator inthis example is available from Optronic Laboratories, Inc, but may besubstituted with other suitable devices.

A silicon detector 32 in FIG. 3 is used to record intensity of reflectedlight L_(R). An exemplary detector is available from OptronicLaboratories, Inc. under the designation “DH-300EC, OL750-HSD-301EC”.Spectra (see, e.g., FIG. 6) are obtained by appropriate software such asthat available from Optronic Laboratories, Inc. The spectra are analyzedusing, for example, IGOR Pro, Version 4.01, available from Wavemetrics,Inc. of Lake Oswego, Oreg.

All electrochemical experiments in the examples detailed below arecarried out using an EG&G PARC Model 263 potentiostat connected with ageneral purpose interface bus (GPIB), available from NationalInstruments of Atlanta, Ga. The GPIB is connected to a Gateway 2000Model P5-60 computer with EG&G Model 270 Research ElectrochemistryPowersuite Software. Gold wire electrodes are available from CHInstruments of Austin, Tex. Potassium ferricyanide is available fromMallinckrodt of Hazelwood, Mo., and sodium sulfate is available fromFisher Scientific of Suwanee, Ga. The potassium ferricyanide and sodiumsulfate are all reagent grade and are used without further purification.All solutions were prepared with deionized water. SEM images werecollected using a Quanta 200 scanning electron microscope available fromFEI Company of Hillsboro, Oreg.

Those skilled in the art will recognize that the foregoing equipment andmaterials are provided by way of example only and many suitablesubstitutions can be made to practice the invention as described below.

The present invention may be better understood with respect to thefollowing examples.

EXAMPLES

With reference to the figures and for exemplary purposes only, thefollowing methodologies and embodiments are employed for interrogationof a solution phase within pores of the film in the ORTLE 10.

Example 1 Characterization of the ORTLE

FIG. 4 depicts results of the ORTLE 10 used as a working electrode. Asshown, a comparison is made between a cyclic voltammogram (CV) obtainedat a conventional gold wire electrode (A) and one obtained using theORTLE 10 as a working electrode (B). Both experiments were carried outin 0.01 M ferricyanide/0.05M sodium sulfate solutions at 20 mVs⁻¹.According to FIG. 4, the ORTLE 10 behaves similarly to the wireelectrode (A). The purely electrochemical characteristics of the ORTLEelectrode reflect those of bulk solution conditions because theelectrode 10 is immersed in bulk solution. This experiment shows theability of the ORTLE 10 to exhibit the standard characteristics thatwould be expected in a ferricyanide solution given that the design ofthe electrode 10 is such that the very thin gold layer 16 sputtered onthe porous alumina film 14 does not seal the pores 18 as previouslydescribed.

More specifically, FIG. 4 shows a decrease in the separation of thepeaks for the ORTLE 10, which would be expected for a contribution fromrestricted diffusion occurring within the pores 18. To test this, anexperiment was conducted to measure the effect of sweep rate (ν) on thepeak current. For a thin layer cell, the peak current should be directlyproportional to ν. This experiment revealed a poor relationship betweenpeak current and ν and an excellent relationship between peak currentand ν^(1/2) (R²=0.999). The result indicates that for this CV thecontribution of thin layer electrochemistry is negligible compared tothat of the bulk solution.

Turning to FIG. 5, a plot shows transmitting spectra of a plain floatglass microscope slide (A), of a similar glass slide with a layer ofporous alumina (B), and of an ORTLE (C), all taken at an angle θ, e.g.,about 45°, to an incident beam (see, e.g., L_(I) in FIG. 3).Interference effects in the spectrum (B) are the result of therefractive index contract between glass and the overlying films, whilethe reduced overall transmittance of the ORTLE is negligible on thescale of FIG. 5 due to the presence of the reflective gold film and thuslittle interrogation of the bulk solution can occur through the film.Spectroscopic changes based on specular reflectance on the backside ofthe gold film must therefore be ascribed to either changes in the mediumwithin the pores or changes in the electrode itself.

FIG. 6 shows a typical specular reflectance spectrum (solid black line)obtained for the ORTLE 10 without any solution in the cell (see, e.g.,cell 20 in FIG. 2). The specular reflectance measurements were performedby positioning the cell at angle θ, e.g., about 25° to 75°, moreparticularly 45°, to the incident beam L_(I), and the detector 32 atabout 80° to 95°, more particularly 90°, to the specular reflected beam(see FIG. 3). By positioning the cell 20 and the detector 32 atrelatively steeper angles, pathlength and sensitivity of the reflectedbeam are increased. Further, steeper angles introduce polarizationeffects that can be used for internal calibration of the components.

With continued reference to FIG. 6, single-beam reflectance measurementswere referenced to the total intensity of the source by directing 100%of the incident beam to the detector before each experiment. Severalsmall interference peaks can be observed at shorter wavelengths with arelatively large peak usually observed within the range of approximately700-1000 nm (the wavelengths and the appearance of the peaks varied fromORTLE to ORTLE due to slight differences in the thickness of theoriginal aluminum films). Upon the introduction of a sodium sulfatesolution, the peaks shifted towards longer wavelengths, accompanied byan increase in intensity—for this sample the large peak shifted from 850to 900 nm, as indicated by the spectrum collected after 0 min (i.e., itwas collected immediately after the introduction of the sodium sulfatesolution). The spectra collected after 60 min and 120 min show that forthis ORTLE no further red shifts or increases in intensity wereobserved. While this particular ORTLE responded promptly, some ORTLEsshowed a gradual red shift over time after solution was introduced. Inall cases, less than 1 hour was necessary for this change to becompleted. At least part of this change is likely the result of poresbeing filled with solution and changing the effective refractive indexof the porous film, as the shift is consistent with changes ininterference fringe positions expected in that case within a factor oftwo.

Referring now to FIG. 7, one condition for the appearance of stronginterference-based oscillations in the reflection spectrum of the ORTLE10 is that the reflectivity of a glass/alumina interface 15 and analumina/gold interface 17 are of comparable magnitudes. This is achievedonly because the electrochemical synthesis of the porous alumina film 14leaves a small amount of aluminum metal behind at the glass/aluminainterface 15, approximately 1.2 nm thickness on average as indicated byellipsometry. Aluminum is the most opaque metal in the visible region.According to optical modeling of this electrode, too much aluminum atthe interfaces 15,17 (e.g., 50 nm) would cause the spectrum of the ORTLE10 to be that of an aluminum mirror. Too little aluminum at theinterfaces 15, 17 would produce a spectrum like that of a gold mirrorwith small (e.g., 5%) interference oscillations in the blue andultraviolet, dropping to about 1% oscillation in the red and nearinfrared.

With this first condition met, constructive interference in reflectionoccurs when the difference in the optical pathlengths a and b are anintegral number of wavelengths of incident light. Assuming an isotropicmaterial with no absorbance, the optical pathlength is the physicalpathlength multiplied by the (real) refractive index of the medium.Making use of Snell's law:Sin(θ₀)=n ₁ Sin(θ₁)=n ₂ Sin(θ₂)where θ₀ is the angle of the incident light L_(I) from air to the glasssubstrate 12, θ₁ is the angle that the beam enters the porous aluminalayer 14 from the glass substrate 12, θ₂ is the angle of the beam thatinteracts with the gold layer 16 and n₁ and n₂ are refractive indices ofthe glass 12 and porous alumina layer 14, respectively.

It is possible to show that the optical path difference (OPD) is:OPD=2d√{square root over (n₂ ²−Sin²(θ₀))}=mλ _(max)where m is a non-negative integer, d is the film thickness and λ_(max)is the maximum wavelength. The apparent refractive index of the film,n₂, is approximately related to the volumetric composition of the films,assuming the film structure to be heterogeneous on a scale less than thewavelength f light and with no regular repeating patterns.

For the dry film,n₂≈n_(Al) ₂ _(O) ₃ (1−f_(p))+f_(p)where n_(Al) ₂ _(O) ₃ is the refractive index of alumina, f_(p) is thepore fraction of the porous alumina and the refractive index of air istaken to be 1. As the pores of the alumina film fill with solutionEquation 3 becomes:n₂≈n_(Al) ₂ _(O) ₃ (1−f_(p))+1.33 f_(p)where the refractive index of the filling solution is assumed to be thatof water. For any value of m,

$\frac{\lambda_{\max,{dry}}}{\lambda_{\max,{wet}}} = \frac{\sqrt{n_{2,{dry}}^{2} - {\sin^{2}( \theta_{0} )}}}{\sqrt{n_{2,{wet}}^{2} - {\sin^{2}( \theta_{0} )}}}$Since the refractive index of the solution filled film is always greaterthan that of the dry film, the filling of the pores will always resultin a red shift.

Example 2 Reduction of Water

In a further aspect of the invention, experiments by specularreflectance spectroscopy on the ORTLE as a function of potential in asimple solution of 0.05 M Na₂ SO₄ provided the following results. Theexemplary embodiment described below is similar to the foregoingembodiment; therefore, those skilled in the art will refer to theprevious embodiment for enabling descriptions of similar components,construction and operation of the ORTLE.

For these experiments, the auxiliary and reference electrodes 28, 30 asshown in FIG. 2 were inserted into the cell 20 and the three electrodes10, 28, 30 were connected to the potentiostat (not shown). FIG. 8 showsa CV of the Na₂ SO₄ solution where the potential was swept from +0.4V to−1.5V vs. (Ag/AgCl). The positive potential limit observed for the ORTLE10 was apparently due to the gold oxide formation. More positivepotentials resulted in delamination of the fragile gold film 16 from theporous alumina substrate 14, a common indicator of stress at afilm/substrate interface 34. The negative potential limit of the ORTLE10 was not due to delamination but apparently to dissolution of theporous alumina 14 by hydroxide ions generated during hydrogen evolution.The cathodic current associated with hydrogen evolution can be seen tobegin at approximately −1.1V (vs. Af/AfCl) in FIG. 8.

An ORTLE 10 for which results are depicted in FIG. 9A show a “dry”reflectance spectrum in which one of the observed peaks was centered at725 nm. The addition of the sodium sulfate solution to the cell 20caused this peak to shift to 755 nm. Reflectance spectra were acquiredduring the 400 s that the potential was held at a certain value forsteps to potentials between +0.1V and −1.2V (vs. Ag/AgCl). FIG. 9A showsdetail in the reflectance spectrum of the ORTLE 10 at a subset of thesepotentials, between −0.4V and −1.1V (vs. Ag/AgCl), with the “dry”spectrum for reference.

No significant changes were observed in the ORTLE reflectance spectra atpotentials positive of −0.5V (vs. Ag/AgCl). When the potential wasstepped to −0.5V (vs. Ag/AgCl), however, a blue shift of the peaks wasobserved. As FIG. 9A shows, the interference peak at 755 nm continued toshift toward the blue very gradually with increasing negative potentialuntil −1.0V. A decrease in intensity was not observed over thispotential range; in fact, a slight increase in intensity was observed.When the potential was stepped to −1.1V, however, a more pronounced blueshift of the large peak at 755 nm was observed that was accompanied by asubstantial decrease in intensity. This is interpreted as a result ofgas infiltrating the pores of the alumina. Spectra obtained at morenegative potentials showed weaker and broader interference peaks thatdid not recover when the electrode was returned to more positivepotentials. This is interpreted as a result of the concomitantgeneration of hydroxide during hydrogen evolution. It is possible thatthe pH may be controlled with a buffer solution.

In electrochemical experiments carried out with ORTLEs outside of thespectrometer, it was not possible to see hydrogen evolution with thenaked eye until the potential approached −1.5V. The increase in currentobserved in FIG. 8 near −1.1V is, however, attributable to the onset ofwater reduction. The low level of hydrogen production occurring at −1.1Vwas insufficient to form bubbles large enough to be observed by thenaked eye, but sufficient to strongly perturb the ORTLE specularreflectance. Assuming pores to be filled initially with water and thatthis water is displaced by gaseous hydrogen generated at the electrode,the apparent refractive index of the film can be written as:

$n_{2} \approx {{n_{A\; l_{2}O_{3}}( {1 - f_{p}} )} + {1.33f_{p}} - \frac{{RTn}_{H_{2}}}{3\;{dA}}}$where n_(H) ₂ is the number of moles of H₂ produced, A is the area ofthe electrode, R is the ideal gas constant, and T is the absolutetemperature.

Inserting this definition into equation 2, solving for wavelength andtaking the derivative with respect to the number of moles of H₂ producedunder initial conditions of pores filled with only water or a water-likeelectrolyte, the following is obtained:

$\begin{matrix}{\lambda_{\max} = \frac{2d\sqrt{( {n_{A\; l_{2}O_{3}} - {f_{p}( {n_{A\; l_{2}O_{3}} - 1.33} )}} )^{2} - {{Sin}^{2}( \theta_{0} )}}}{m}} \\{\frac{\partial\lambda_{\max}}{\partial n_{H_{3}}} = \frac{2{{RT}( {n_{A\; l_{2}O_{3}} - {f_{p}( {n_{A\; l_{2}O_{3}} - 1.33} )}} )}}{3{mA}\sqrt{( {n_{A\; l_{2}O_{3}} - {f_{p}( {n_{A\; l_{2}O_{3}} - 1.33} )}} )^{2} - {{Sin}^{2}( \theta_{0} )}}}}\end{matrix}$

Inserting a void fraction of 0.32 (estimated from ellipsometry), a filmthickness d of 680 nm (estimated from modeling of the film in FIG. 6),and an incident angle of 45 degrees, the maxima should occur at:

$\lambda_{\max} \approx \frac{1851\mspace{11mu}{nm}}{m}$

From Equation 5, it is evident that m=2 for the long wavelength peak inFIG. 6, and m is 3, 4 and 5 for the peaks at progressively shorterwavelengths. Returning to Equation 4, the sensitivity of the peakposition of the m=2 peak to hydrogen is:

$\frac{\partial\lambda_{\max}}{\partial n_{H_{2}}} = \frac{9 \times 10^{10}\mspace{11mu}{{nm} \cdot {mol}^{- 1} \cdot {cm}^{2}}}{A}$

Equation 6 indicates that 0.1 nanomole of H₂ produced per centimetersquared area of electrode surface could result in a 9 nanometerhypsochromic shift in the m=2 peak. Assuming that a 1 nm shift could bedetected, and assuming the sensor on the end of a fiber-optic with anarea of 10⁻⁴ cm, approximately 1 fM of H₂ evolution could be detected ifthe gas were captured in the pores of the alumina.

Subtle shifts in the wavelength of the interference peak maximum in FIG.9A were observed at potentials as positive as −0.5V (vs. Ag/AgCl). Theorigin of this shift is unknown at present, but must involve changes inthe composition of the porous film or of the solution in the porevolume. The blue shift indicates an overall decrease in the opticalthickness of the film under these conditions, an effect that isconsistent with displacement of pore solution by nanoscale bubbles. InFIG. 9B, the variation in the magnitude of the reflectance at 780 nm isplotted against potential. As shown, the intensity follows a similartrend with varying potential, in that a large increase of intensity isobserved upon the addition of solution and a gradual decrease observedwith increasing negative potential. Eventually, at −1.1V, FIG. 9B showsthat the intensity is similar to that of the electrode in the dry state.

CONCLUSION

A thin layer film based on a gold-coated, porous alumina film, forexample, has been shown and described. The thin layer film was placed ina cell and subjected to cyclic voltammetry and spectroscopic techniquesand by spectroelectrochemistry in which a combination of specularreflectance spectroscopy and chronoamperometry was used. Typical spectraexhibited several strong interference peaks that resulted from thepresence of a small amount of unanodized aluminum at the glass/porousalumina interface. A red shift of the peaks in the specular reflectancespectrum and an increase in intensity was observed upon the introductionof a sodium sulfate solution to the spectroelectrochemical cell wherethe ORTLE was mounted. This is likely due to refractive index changesarising from the filling of the pores by the solution. A blue shift ofthe peaks could be induced by stepping the potential to valuesincreasingly negative of −0.5V (vs. Ag/AgCl) and towards the backgroundlimit of the solution. Upon stepping to −1.1V, a pronounced blue shiftwas observed, accompanied by a decrease in intensity. This is likely dueto the production of hydrogen within the pores of the ORTLE.

The ORTLE differs from typical thin-layer electrodes in several ways.First, no transparent electrodes are required, and the solution does nothave to be transparent or even homogenous, because the nanostructuredelectrode face filters out particles large enough to cause significantscattering. The electrode can thus be used in bulk solutions as a windowthat does not allow light into the bulk—similar to total internalreflection techniques, but with no critical angle constraints. Further,the electrode can potentially be designed to combine refractive-indexmeasurements with surface plasmon resonance and UV-visible absorbancemeasurements with very minor changes. Moreover, while a top layer of theORTLE can be a metal, the top layer is not limited to use as anelectrode for spectroelectrochemical analysis. Additionally, while theinterference pattern shifts described above indicate a change in therefractive index of the material filling the pores, measurements are notlimited to interferometry but can include absorption properties of thefluid. Thus, one can also use the ORTLE to perform infrared orUV-visible spectroscopy measurements on the fluids in the pores toidentify those fluids.

While preferred embodiments of the invention have been shown anddescribed, those skilled in the art will recognize that other changesand modifications may be made to the foregoing examples withoutdeparting from the scope and spirit of the invention. It is intended toclaim all such changes and modifications as fall within the scope of theappended claims and their equivalents. Moreover, references herein to“top,” “bottom,” “upward,” “upper,” “higher,” “lower,” “downward,”“descending,” “ascending,” “side,” “first,” and “second” structures,elements, designations, geometries and the like are intended solely forpurposes of providing an enabling disclosure and in no way suggestlimitations regarding the operative orientation or order of theexemplary embodiments or any components thereof.

1. A method for analyzing matter comprising the steps of: (a)introducing a liquid or gaseous matter into an optically reflective thinlayer electrode, the electrode including a transparent base substratewith alumina film disposed thereon, the alumina film defining aplurality of pores therein, and a gold film disposed on the alumina filmsuch that a quantity of the liquid or gaseous matter can enter at leastone of the pores; (b) applying a potential to the gold film such thatthe quantity of the liquid or gaseous matter in the pores is isolatedfrom a remaining bulk of the liquid or gaseous matter disposed about theelectrode; (c) directing light from a source into the electrode fromproximate the base substrate in a direction of the gold film, the goldfilm under the potential configured to reflect the light into thequantity of the liquid or gaseous matter in the pores for analysis ofthe reflected light; and (d) monitoring the reflected light by taking areflectance measurement.
 2. The method as in claim 1, wherein the liquidor gaseous matter is selected from the group consisting of potassiumferricyanide, sodium sulfate, water and solutions thereof.
 3. The methodas in claim 2, wherein the liquid or gaseous matter is a solution of0.01M ferricyanide, 0.05M sodium sulfate, and deionized water.
 4. Themethod as in claim 1, wherein the transparent base substrate is made ofglass.
 5. The method as in claim 1, wherein the applied potential isbetween +0.4V to −1.5V.
 6. The method as in claim 1, further comprisingthe step of holding the potential for 200 seconds to 400 seconds.
 7. Themethod as in claim 1, further comprising the step of directing the lightfrom the source at the base substrate at about a 45° angle.
 8. Themethod as in claim 1, further comprising the step of monitoring thereflected light in the quantity of the liquid or gaseous matter isolatedin the pores by reflectance spectroscopy.
 9. The method as in claim 1,further comprising the step of talking specular reflectance measurementsby a detector disposed at about 90° to the reflected light.
 10. Anoptically reflective thin layer, comprising: a transparent basesubstrate; a film disposed on the base substrate, the film defining aplurality of pores therein; and a reflective material disposed on thefilm such that the pores are exposed to atmosphere, the reflectivematerial having a specular surface for reflection of light into thepores for taking measurements of a fluid isolated therein from theatmosphere; wherein the reflective material is a specular gold filmconfigured to reflect a light beam that has been transmitted from adirection of the base substrate into the quantity of fluid disposed inthe pores; wherein the quantity of fluid in the pores is monitored byspecular reflectance spectroscopy, spectroelectrochemical analysis,interferometric analysis or combinations thereof.
 11. The opticallyreflective thin layer as in claim 10, wherein the transparent basesubstrate is glass.
 12. The optically reflective thin layer as in claim10, wherein the pores are from 80 nm to about 100 nm in diameter anddefine a depth from 250 nm to about 1000 nm, the pores configured tohold a quantity of fluid when the optically reflective thin layer isimmersed therein.
 13. The optically reflective thin layer as in claim12, wherein the fluid is selected from the group consisting of aferricyanide, a sodium sulfate, a water, a gas, and combinationsthereof.